In general, a sheet handling system inspects and processes sheets such as bank notes or bills to determine whether the bills are genuine or forged and whether they are usable or discarded. Thus, it is quite important to detect overlapped sheets. For that purpose, an overlapped-sheet detection apparatus is installed in the sheet handling system.
A conventional overlapped-sheet detection apparatus is disclosed in Japanese Unexamined Patent Publication Tokkaihei 7-10322. First, the overlapped-sheet detection apparatus measures a length of sheets along a conveying direction at its entrance. The overlapped-sheet detection apparatus is provided with two conveying belts driven at different speeds. The first and second conveying belts are in contact with the surface and back of the sheets, respectively. Since the sheets are conveyed at different speeds, they are separate from each other. Then, the overlapped-sheet detection apparatus measures again a length of sheets along the conveying direction at its exit. If the data measured satisfy the following equation, the overlapped-sheet detection apparatus judges overlapped sheets:(Lout−Lin)≧Ld where “Lin” is a length of sheets measured at the entrance, “Lout” is a length of sheets measured at the exit, and “Ld” is a predetermined judgment level of overlapped sheets.
However, the overlapped-sheet detection apparatus, cannot always detect overlapped sheets correctly in the case where the difference in conveying speeds for the first and second conveying belts is small because the overlapped sheets are not so readily separate as they are supposed to be. If the difference of conveying speeds is set to be higher to overcome such a problem, the first and second conveying belts are worn out short of their expected life.